In Redundant Arrays of Independent Disk (RAID) systems, data files and related parity are striped across multiple disk drives. In storage subsystems which manage multiple hard disk drives as a single logical direct access storage device (DASD), the RAID logic is implemented in the controller of the subsystem.
RAID storage methodologies have also been implemented in software for execution on a single host computer. This allows the single host computer, such as a personal computer, to implement RAID storage techniques on local hard disk drives.
One challenge in shared disk systems implementing a parallel, shared disk RAID architecture is to provide a system for insuring that data and parity data are properly updated to disks in the system.
The Integrated Sector Format (ISF) disk drive is a relatively new standard for improving the error correcting capabilities of a disk drive. Current disk drives have a single level of error correcting code (ECC) for each individual sector. ISF improves this architecture by grouping every eight (8) consecutive sectors into a cluster and adding a second and a third level of ECC to each cluster. The third level ECC is computed over all 8 sectors. These two levels of additional ECC's provide the additional error correction capabilities above and beyond those provided by the first level of ECC. An exemplary ISF method is described in U.S. patent application, titled “Multiple Level (ML), Integrated Sector Format (ISF), Error Correction Code (ECC) Encoding And Decoding Processes For Data Storage Or Communication Devices And Systems,” Ser. No. 10/040,115, filed on Jan. 3, 2002, supra.
In an ISF disk drive when a fraction of a cluster is updated (i.e., written), the drive first reads the cluster into its buffer memory, inserts the new sectors that are updated, re-computes the new ECCs, and then writes the cluster back to the disk media. This process which is referred to as a read-modify-write action, adds one revolution to the time of the write command at a performance penalty.
The most common RAID systems are based on parity schemes to provide added fault tolerance. For illustration purpose only, the RAID-5 system will be described to illustrate the invention, with the understanding that other parity-based disk array systems may be used.
In a RAID-5 system, to update (or write) a small piece of data, the RAID controller first reads the old data in that location, reads the corresponding old parity from the corresponding parity drive, and XOR (exclusive OR) them with the new data to generate the new parity, after which it can write the new data to the data drive and the new parity to the parity drive. In other terms, the RAID controller needs to do read-modify-write of the data drive and the parity drive. Again, the read-modify-write procedure imposes a performance penalty to the write command execution.
In the combination of ISF disk drives used in a RAID-5 system, it could take for example a total of six disk accesses to complete a write command: three accesses to the data drive and three other accesses to the parity drive. What is therefore needed is a system and associated method for minimizing the accesses to the ISF data drive to complete a write command.